In CT X-ray imaging of a patient, X-rays are used to image internal structure and features of a region of the person's body. The imaging is performed by a CT-imaging system, hereinafter referred to as a “CT-scanner” that images internal structure and features of a plurality of contiguous relatively thin planar slices of the body region using X-rays.
The CT-scanner generally comprises an X-ray source that provides a planar, fan-shaped X-ray beam and an array of closely spaced X-ray detectors that are coplanar with the fan beam and face the X-ray source. The X-ray source and array of detectors are mounted in a gantry so that a person being imaged with the CT-scanner, generally lying on an appropriate support couch, can be positioned within the gantry between the X-ray source and the array of detectors. The gantry and couch are moveable relative to each other so that the X-ray source and detector array can be positioned axially at desired locations along the patient's body.
The gantry comprises a stationary structure referred to as a stator and a rotary element, referred to as a rotor, which is mounted to the stator so that the rotor is rotatable about the axial direction. In third generation CT-scanners the X-ray source and detectors are mounted to the rotor. Angular position of the rotor about the axial direction is controllable so that the X-ray source can be positioned at desired angles, referred to as “view angles”, around the patient's body.
To image a slice in a region of a patient's body, the X-ray source is positioned at the axial position of the slice and the X-ray source is rotated around the slice to illuminate the slice with X-rays from a plurality of different view angles. At each view angle, detectors in the array of detectors measure intensity of X-rays from the source that pass through the slice. The intensity of X-rays measured by a particular detector in the array of detectors is a function of an amount by which X-rays are attenuated by material in the slice along a path length from the X-ray source, through the particular slice, to the detector. The measurement provides information on composition and density of tissue in the slice along the path-length.
For example, let incident X-ray intensity sensed by an “n-th” detector in the array of detectors when the X-ray source is located at a view angle θ is represented by I(n,θ), then I(n,θ)=IOexp(−∫μ(l)dl). In the expression for I(n,θ), IO is intensity of X-rays with which the X-ray source illuminates the slice, integration over l represents integration over a path through material in the slice along a direction from the X-ray source to the n-th detector and μ(l) is an absorption coefficient for X-rays per unit path-length in the material at position l along the path. (Dependence of the integral on n and θ is not shown explicitly and is determined through dependence of the length and direction of the path-length l on n and θ.)
From IO and the sensed I(n,θ), an amount by which X-rays are attenuated along path-length l and a value for the integral ∫μ(l)dl, hereinafter referred to as an “line integral”, can be determined. The attenuation measurement provided by the n-th detector at the view angle θ therefore provides a value for the line integral of the absorption coefficient along a particular path length through the slice which is determined by θ and the known position of the n-th detector relative to the X-ray source.
The set of attenuation measurements for a slice provided by all the detectors in the detector array at a particular view angle θ is referred to as a view. The set of attenuation measurements from all the views of the slice is referred to as a “projection” of the slice. Values for the line integral provided by data from the projection of the slice are processed using algorithms known in the art to provide a map of the absorption coefficient μ as a function of position in the slice. Maps of the absorption coefficient for the plurality of contiguous slices in the region of the patient's body are used to display and identify internal organs and features of the region.
In some CT-scanners, to image a region of a patient, a sequential scan of the patient is performed in which the region is scanned by moving the patient stepwise in the z direction to “step” the region through the gantry that houses the X-ray source and detector array. Following each step, the X-ray source is rotated through 360 degrees or (180+Δ) degrees, where Δ is an angular width of the fan beam provided by the X-ray source, to acquire a projection of a slice of the region. In some CT-imagers a “spiral scan” of a patient is performed in which the region of the patient is steadily advanced through the gantry while the X-ray source simultaneously rotates around the patient and projections of slices in the region are acquired “on the fly”. In some CT-scanners, referred to as multislice CT-scanners, a plurality of slices of a region of a patient are simultaneously imaged. Often as many as four slices of a region of a patient are simultaneously imaged by a multislice CT-scanner.
In third generation CT-scanners the X-ray source and the detectors are mounted on the imager's rotor. Data generated by the detectors responsive to intensity of X-rays incident on the detectors has to be transferred from the rotor to a location of a processor that generates images from the data and displays the generated images.
Many different methods and systems are available for transmitting data generated by detectors on the rotor of a CT-scanner to a desired location for processing and display. However, the immediate environment of the rotor is generally electromagnetically very noisy. As a result, free space transmission of the data using electromagnetic waves has not been considered practical. Usually, data generated by the detectors is transferred from the rotor over very short distances to the stator via contact connections or non-contact “proximity” connections between the rotor and stator. From the stator the data is transmitted via wire or optical fiber to a desired location where the data is processed and/or displayed.
In some third generation CT-scanners electromechanical, contact slip-rings provide contact connections between the imager's rotor and stator for transmitting data from the rotor to the stator. However, present day CT-scanners generate data at data rates between 20-800 Mbits/s and data rates are increasing as CT-scanners become faster and multislice CT-scanners acquire projections of an increasing plurality of slices simultaneously. Contact slip-rings generally cannot support reliable data transfer at data transfer rates that match rates at which modern third generation CT-scanner generate data.
Usually therefore, in modem CT-scanners data is transmitted between the rotor and stator via non-contact proximity links, which links may, for example, be optical or electromagnetic. Various types of such non-contact links are commercially available, for example, from Schliefring (Germany), Litton Poly-Scientific (USA) and ElectroTech (USA). However, currently available non-contact links for CT-scanners are generally expensive and they usually complicate mechanical construction of the imagers. Furthermore, present non-contact links generally cannot support data transfer rates about equal to or greater than 1 Gbit/sec.
U.S. Pat. No. 5,577,026 describes a non-contact data link for transferring data between the rotor and stator of a CT-scanner in which the data is transferred via antenna assemblies on the rotor and stator. In an embodiment of the data link the antenna assemblies are capacitively coupled and data is transferred between the antenna using RF signals. The inventors note that IR, UV or optical frequencies may also be used to transfer data. The inventors describe coding data transmitted over the link using an Ethernet protocol and achieving reliable data transfer rates of approximately 10 megabits per second (Mbits/s).
U.S. Pat. No. 5,530,425 describes a system for transmitting data from a CT-scanner rotor to the imager stator using a transmission line mounted on the rotor and a non-contact proximity RF coupler mounted on the stator which is coupled to the transmission line. The inventor notes that the system supports data transmission rates at 150 Mbits/s.
U.S. Pat. No. 5,469,488 describes an optical system for transmitting data between the rotor and stator of a CT-scanner. A plurality of light emitting elements in the system is located on the rotor or stator and transmits optical signals to a light receiving element mounted respectively on the stator or the rotor to transfer data between the stator and rotor.